1. Field of the Invention
Exemplary embodiments of the present invention relate to a light emitting device and a method of fabricating the same, and more particularly, to a light emitting device having a reflective metal layer and a method of fabricating the same.
2. Discussion of the Background
In general, since Group-III-element nitride semiconductors, such as GaN and AlGaN, have excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as materials for light emitting devices in blue and ultraviolet regions. Particularly, blue and green light emitting devices using GaInN are used in various applications such as large-sized full-color flat panel displays, backlight sources, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, and optical communications.
Since it is difficult to produce a homogeneous substrate for enabling such a Group-III-element nitride semiconductor to be grown thereon, the nitride semiconductor is grown on a heterogeneous substrate having a crystal structure similar to that of the nitride semiconductor through a process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). A sapphire substrate having a hexagonal system crystal structure is frequently used as the heterogeneous substrate.
Since a sapphire substrate used as the heterogeneous substrate is insulative, a light emitting device having a horizontal structure is fabricated, in which electrode pads are all positioned on top of the substrate, and a p-type GaN layer is positioned in an upper portion of the light emitting device. The p-type GaN layer is formed relatively thin because of its high resistance caused by a limit of epitaxial growth. Transparent electrodes and pads for current spreading are generally formed on the p-type GaN layer. In a large area light emitting device, branch lines extending from pads on a p-type and/or an n-type GaN layers are formed to spread current throughout a wide area. Meanwhile, a reflective metal layer is generally formed on the bottom surface of the sapphire substrate to reflect the light that travels toward the lower portion of the light emitting device.
However, as transparent electrodes and pads, which are employed in a conventional light emitting device, and branch lines extending from the pads are formed on a light emission surface, they absorb the light emitted from an active layer, whereby a light emitting efficiency is decreased. Further, the reflective metal layer is relatively quite distant from the active layer, and hence, a large amount of light may be lost until the light is reflected from the reflective metal layer and emitted to the outside.
Techniques for roughening a light emission surface have been studied to improve a light extraction efficiency. However, a p-type GaN layer cannot be formed thick because of its high resistance, creating a limit in forming a light emission surface to be rough.
Meanwhile, AC light emitting diodes have been commercialized, in which light is continuously emitted by connecting light emitting diodes (LEDs) directly to an AC power source. For example, a light emitting diode capable of being directly connected to a high-voltage AC power source is disclosed in PCT Patent Publication No. WO 2004/023568A1 (SAKAI et al.), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS.”
According to PCT Patent Publication No. WO 2004/023568A1, LEDs are two-dimensionally connected on an insulative substrate such as a sapphire substrate to form LED arrays. Such LED arrays are connected to each other in reverse parallel on the sapphire substrate. As a result, there is provided a single-chip light emitting device capable of being driven by an AC power supply.
Since the AC-LED has light emitting cells formed on a substrate used as a growth substrate, e.g., a sapphire substrate, the light emitting cells have a limitation in structure, and there is a limitation in improving light extraction efficiency. To solve such a problem, a method of fabricating an AC-LED using a substrate separation process is disclosed in Korean Patent Publication No. 10-0599012, entitled “LIGHT EMITTING DIODE HAVING THERMAL CONDUCTIVE SUBSTRATE AND METHOD OF FABRICATING THE SAME.”
FIG. 1, FIG. 2, FIG. 3, and FIG. 4 are sectional views illustrating a method of fabricating an AC light emitting device according to a prior art.
Referring to FIG. 1, semiconductor layers comprising a buffer layer 23, an n-type semiconductor layer 25, an active layer 27, and a p-type semiconductor layer 29 are formed on a sacrificial substrate 21. A first metal layer 31 is formed on the semiconductor layers, and a second metal layer 53 is formed on a substrate 51 that is separate from the sacrificial substrate 21. The first metal layer 31 may comprise a reflective metal layer. The second metal layer 53 is joined with the first metal layer 31 so that the substrate 51 is bonded on top of the semiconductor layers.
Referring to FIG. 2, after the substrate 51 is bonded, the sacrificial layer 21 is separated by a laser lift-off process. Also, after the substrate 21 is separated, the remaining buffer layer 23 is removed, and a surface of the n-type semiconductor layer 25 is exposed.
Referring to FIG. 3, the n-type semiconductor layer 25, the active layer 27, the p-type semiconductor layer 29, the first metal layer 31, and the second metal layer 53 are patterned using a photolithography technique so as to form metal patterns 40 spaced apart from one another and light emitting cells 30 positioned on regions of the respective metal patterns 40. Each of the light emitting cells 30 comprises a patterned p-type semiconductor layer 29a, a patterned active layer 27a and a patterned n-type semiconductor layer 25a. 
Referring to FIG. 4, metal wires 57 are formed to electrically connect top surfaces of the light emitting cells 30 to the metal patterns 40 adjacent thereto. The metal wires 57 connect the light emitting cells 30, thereby forming a serial array of light emitting cells. Electrode pads 55 for connecting the metal wires 57 may be formed on the n-type semiconductor layers 25a. Electrode pads may also be formed on the metal patterns 40. Two or more arrays may be formed and these arrays are connected in reverse parallel, so that an LED capable of being driven by an AC power source is provided.
According to the prior art, thermal dissipation performance of the LED can be improved since the substrate 51 can be selected from a variety of substrates, and a light extraction efficiency can be enhanced by treating a surface of the n-type semiconductor layer 25a. Further, the first metal layer 31a comprises a reflective metal layer and reflects light traveling from the light emitting cells 30 toward the substrate 51, so that the light emitting efficiency can further improved.
However, in the prior art, while the n-type semiconductor layer 25, the active layer 27, the p-type semiconductor layer 29, the first metal layer 31, and the second metal layer 53 are patterned, etching byproducts of a metallic material stick or adhere to side walls of the light emitting cells 30 and increasing the chances of a short circuit between the n-type semiconductor layer 25a and the p-type semiconductor layer 29. Further, a surface of the first metal layer 31a, which is exposed while the n-type semiconductor layer 25, the active layer 27, and the p-type semiconductor layer 29 are etched, may be easily damaged by plasma. When the first metal layer 31a comprises a reflective metal layer such as Ag or Al, such etching damage may be serious. Since the surface of the metal layer 31a is damaged by plasma, the adhesion of the wires 57 or electrode pads formed on the metal layer 31a is lowered, resulting in a device failure.
Meanwhile, according to the prior art, the first metal layer 31 may comprise a reflective metal layer, thereby reflecting light traveling from the light emitting cells 30 toward the substrate 51. However, it is difficult to expect that light is reflected in spaces between the light emitting cells 30 due to the etching damage or oxidation of the reflective layer. Further, since the substrate 51 is exposed in regions between the metal patterns 40, light may be lost by being absorbed by the substrate 51.
Furthermore, since the wire 57 is connected to a top surface of the n-type semiconductor layer 25a, i.e., a light emission surface, the light emitted from the active layer 25a is absorbed by the wire 57 and/or the electrode pad 55, thereby also resulting in light loss.